The present invention relates to a microactuator used to drive optical components and small-size magneto-optical/magnetic disk components, and a method of manufacturing the same.
A microactuator (electrostatic actuator) is generally proposed in which a movable element made of an insulating substance is moved by an electrostatic force generated between a plurality of stationary electrodes and the charges induced by the movable element when a voltage is applied to the plurality of stationary electrodes opposing the movable element at a small gap.
A microactuator mounted at the distal end of a suspension supported by an arm in a magnetic disk apparatus to drive a magnetic head formed integrally with a slider is proposed in L.S. Fan et al., "Magnetic Recording Head Positioning at Very High Track Densities Using a Microactuator-Based, Two-Stage Servo System", IEEE Transactions on Industrial Electronics, Vol. 42, No. 3, pp. 222-233, June 1995 (reference 1).
FIG. 9 shows a microactuator described in reference 1.
In FIG. 9, the conventional microactuator is constituted by a pair of T-shaped stationary elements 83 and 84 which are formed on a silicon substrate (to be described later) and have the distal ends of leg portions opposing each other, and an H-shaped movable element 82 formed between the stationary elements 83 and 84. The movable element 82 is supported by four springs 81 to float above the silicon substrate. One end of each spring 81 is fixed to a corresponding one of a pair of spring bases 80 fixed to the silicon substrate, and the entire spring 81 is separated from the silicon substrate.
The stationary elements 83 and 84 are respectively made up of support portions 83a and 84a, and support portions 83b and 84b constituting leg portions vertically extending from the centers of the support portions 83a and 84a. The end portions of the support portions 83b and 84b oppose each other. Many comb tooth portions 91 are formed in a comb tooth shape at a predetermined pitch in two lines on the two sides of each of the support portions 83b and 84b. As shown in FIG. 10, many stationary element electrodes 93 are formed at a predetermined pitch in a comb tooth shape on one side of each comb tooth portion 91.
The movable element 82 is made up of a pair of parallel support portions 82a and a coupling portion 82b coupling the centers of the support portions 82a. The movable element 82 is combined with the stationary elements 83 and 84 to constitute an actuator. That is, the support portions 82a of the movable element 82 are arranged parallel to sandwich the support portions 83b and 84b of the stationary elements 83 and 84. The coupling portion 82b of the movable element 82 vertically crosses the gap formed by the end portions of the support portions 83b and 84b of the stationary elements 83 and 84.
The movable element 82 comprises many comb tooth portions 92 formed in a comb tooth shape at the same pitch as that between the comb tooth portions 91 of the stationary elements 83 and 84. The comb tooth portions 91 of the stationary elements 83 and 84 and the comb tooth portions 92 of the movable element 82 overlap and interdigitated with each other. As shown in FIG. 10, movable element electrodes 94 to be inserted between the stationary element electrodes 93 are formed on one side of each comb tooth portion 92.
As shown in FIG. 11, the comb tooth portion 91 formed integrally with the stationary element electrode 93 is fixed to a silicon substrate 100 via a stationary element base 101. In contrast to this, the comb tooth portion 92 formed integrally with the movable element electrode 94 is separated from the silicon substrate 100, i.e., floats above the surface of the semiconductor substrate 100 at a predetermined interval.
In this arrangement, the movable element 82 can be moved right or left in FIG. 9, i.e., the comb tooth portion 92 can be moved in a direction to come close to and separate from the comb tooth portions 91 by applying a voltage across the movable element electrode 94 of the comb tooth portion 92 and the stationary element electrodes 93 of the stationary elements 83 and 84. In this case, the movable element 82 can be moved left by applying a voltage to the left stationary element 84 in FIG. 9, or right by applying a voltage to the right stationary element 83.
A method of manufacturing the microactuator having this arrangement will be explained. A 2-.mu.m thick PSG (PhoshoSilicate Glass) film is patterned in a region on the silicon substrate 100 where the movable element 82 is to be formed. Copper is plated between resist patterns formed on the PSG film using photolithography.
The PSG film is removed using hydrofluoric acid to separate the movable element 82 including the movable element electrode 94 from the silicon substrate 100, thereby forming the copper-plated movable element 82. In this way, the microactuator in reference 1 using a 20-.mu.m thick copper material is manufactured.
In a microactuator using a silicon IC process, a structure using a polysilicon thin film has conventionally been known well. Compared to the electroplated actuator, the microactuator with a polysilicon structure has good matching with the silicon IC process and exhibits excellent mechanical characteristics. Note that in applications to a magnetic/magneto-optical head and the like, movement of the head in directions other than a desired direction must be suppressed small.
In the microactuator shown in FIG. 9, the movable element 82 must move right and left in FIG. 9, but its movement in a direction perpendicular to the surface of the silicon substrate 100 must be suppressed as small as possible. From this condition, the spring 81 must be made thick. The movable element electrode 94 and the stationary element electrode 93 must also be made thick in order to use a large electrostatic force.
From these conditions, a microactuator having an electrode thickness of 20 .mu.m or more must be manufactured for practical use. Since the polysilicon thin film has a thickness of about 4 .mu.m at most, microactuators using the above-described plating technique and a single-crystal silicon etching technique (to be described later) are being developed.
To manufacture a microactuator made of single-crystal silicon, the method using an SOI (Silicon On Insulator) substrate described in A. Benitez et al., "Bulk Silicon Microelectromechanical Devices Fabricated from Commercial Bonded and Etched-Back Silicon-on-Insulator Substrates", Sensors and Actuators, A50, pp. 99-103, 1995 (reference 2) can be employed.
According to this method, the movable element electrode 94 and the stationary element electrode 93 in FIG. 11 are formed of a 20-.mu.m thick single-crystal silicon film, and the stationary element base 101 is formed of a silicon oxide film. By removing the silicon oxide film positioned below the movable element electrode 94 using hydrofluoric acid, the movable element electrode 94 can be separated from the silicon substrate 100.
In this case, since the movable element electrode 94 is narrower in width than the stationary element electrode 93, the silicon oxide film is still left below the stationary element electrode 93 even upon etching using hydrofluoric acid, and forms the stationary element base 101. In this manner, the movable element electrode 94 and the stationary element electrode 93 each made of, e.g., a 20-.mu.m thick single-crystal silicon film are formed on the silicon substrate 100.
The method of manufacturing a thick microactuator has been briefly described. The conventional microactuator shown in FIG. 9 is undesirably easily destructed by external shock, as will be described below.
I) To enable the microactuator to use a very weak electrostatic energy, the spring 81 is formed of a wire having a width of 2 .mu.m and a length of 200 .mu.m. Accordingly, the restoring force of the spring 81 in the displacement direction of the movable element 82 is very small. Since a slider having a weight of about 1 mg is mounted on the movable element 82, the movable element 82 of the microactuator is easily destructed by even small shock.
The cause of the shock includes collision of the slider against a magnetic disk during the driving of the magnetic head, and sudden contact to the microactuator during the assembly of the microactuator. Also when the microactuator base moves suddenly, an acceleration force acts on to destruct the microactuator because the 1-mg slider is mounted on the movable element 82.
II) A narrow gap having a width of about 2 .mu.m is formed between the opposing surfaces of the movable element electrode 94 and stationary element electrode 93 of the microactuator. The narrow gap is necessary to efficiently use a voltage applied across the two electrodes 93 and 94. However, if a foreign substance such as dirt enters the gap, the microactuator fails to operate. In the conventional microactuator shown in FIG. 9, this operation failure is frequently caused by such a foreign substance.
III) In the conventional microactuator shown in FIG. 9, the movable element 82 can be moved left by applying a voltage across the left stationary element 84 and the movable element 82, or right by applying a voltage to the right stationary element 83. During this driving, the movable element 82 may move not parallel to the surface of the silicon substrate 100 but with an inclination at a certain angle. In this case, the magnetic head is also inclined to the disk surface, so information is erroneously read/written.
In addition, demand arises for structural improvement of a microactuator of this type. More specifically, since the microactuator shown in FIG. 9 has a planar dimension of 2 mm.times.2 mm, many microactuators can be formed from a silicon wafer having a diameter of 150 mm. Increasing the number of actuators formed from one wafer can directly reduce the cost. In the conventional structure, the spring 81 and the spring base 80 project from the device main body. This obstructs arranging a larger number of actuators. From this viewpoint, the structural improvement of the actuator is eagerly demanded.